1. Field of the Invention
The present invention relates to a damper for use in a loud speaker to be used for various acoustic apparatuses, and a method for producing the same.
2. Description of the Related Art
FIG. 1 is a half cross-sectional view showing a configuration for a typical loud speaker 20. FIG. 2 is an exploded perspective view showing details of the loud speaker 20. The same constituent elements are indicated by the same reference numerals in FIGS. 1 and 2.
As shown in FIGS. 1 and 2, the loud speaker 20 includes a lower plate 3 integral with a center pole 2, a magnet ring 4 provided on a bottom portion of the lower plate 3 so as to surround the center pole 2, and an upper plate 5 provided on an upper face of the magnet ring 4. The lower plate 3, the magnet ring 4, and the upper plate 5 are coupled to one another to constitute a magnet circuit 1.
On an upper face of the upper plate 5, an inner periphery of the frame 6 is coupled. A gasket 7 and an outer periphery of a diaphragm 8 are attached to an outer periphery of the frame 6 by using an adhesive. A voice coil 9 is coupled to an inner periphery of the diaphragm 8.
A middle portion of the voice coil 9 is supported by an inner periphery of the damper 10, an outer periphery of the damper 10 being supported by the frame 6. A lower portion of the voice coil 9 is inserted into a magnetic gap 11 formed between the center pole 2 of the lower frame 3 and the upper frame 5 (which are included in the magnetic circuit 1) without being eccentric. Moreover, a dust cap 12 for preventing dust from entering the magnetic circuit 1 is provided on the upper side of a central portion of the diaphragm 8.
The damper 10 functions as a support for the voice coil 9. That is, the damper 10 functions to prevent the voice coil 9 from making unfavorable movements, e.g., excessive vibration or rolling, even when an excessive vibration is applied to the voice coil 9.
The damper 10 is conventionally produced by forming a prepreg, which serves as a substrate, into a predetermined shape by using a heated mold. The prepreg is formed by impregnating a fabric matrix composed of cotton yarn, aramid fiber yarn, phenol fiber yarn, or a blended yarn thereof with a thermosetting resin such as phenol resin or melamine resin as an excipient.
However, the conventional damper produced in the above-mentioned manner, or the producing method itself, has the four following problems.
First, the efficiency of the production method is not optimized. In the above-described conventional method for producing a damper, a step for forming a prepreg by impregnating a fabric with an excipient is required. Solutions such as phenol resin and melamine resin, which are used as excipients in this step, may act on the skin of a person engaged in the production thereof to cause a rash or may generate poisonous gases when dried, thereby hindering work efficiency.
Second, deformation of the damper during the production process may occur. In the above-mentioned production method, the excipient included in the prepreg is a thermosetting resin, which is to be cured by a thermal reaction in a mold heated at a predetermined temperature into a predetermined shape. On the other hand, the fabric included in the prepreg is composed of natural fibers such as cotton yarn, or heat-resistant artificial fibers such as aramid fibers or phenol fibers, and therefore is hardly deformed during the heating process using the heated mold. In other words, the shape of the damper is conserved by the excipient. However, the damper is liable to deform during the production process for the following reasons. In order to reduce the time required for molding, the mold is usually heated at a relatively high temperature, e.g., 180.degree. C. or more. As a result, the damper set in the mold cannot be sufficiently cooled after the curing reaction terminates, so that it is still in a relatively soft, rubber-sheet like state. When one attempts to remove the molded damper in this state from the mold, the damper may not retain the predetermined shape due to the internal stress of the fabric having relatively high stiffness, and consequently is often deformed.
Third, the durability of the damper as a constituent element of a loud speaker may be inadequate. The function of a molded damper results in it being repeatedly deformed through flexure and bending. Since the phenol resin, melamine resin, and the like used as excipient materials have relatively low comformability with the fibers constituting the fabric, peeling may occur at interfaces between the fabric and the excipient through use over time. Moreover, although the excipient (such as phenol resin or melamine resin), which coats over the surface of the fibers of the fabric in the form of a relatively thin film, maintains a very high elasticity when cooled to room temperature after the molding, it has a low internal loss and, consequently, relatively high fragility. As a result, the thin film of excipient may not withstand the flexure of the fabric having high flexibility and accordingly be ripped. In that case, the bonds at the intersections of the fibers of the fabric are destroyed, greatly reducing the stiffness of the entire damper.
Fourth, the water-proofness of the damper may be inadequate. Dampers to be used for loud speakers attached on the doors of automobiles are required to have little deformation against repetitive moistening and drying. However, the above-mentioned resin materials constituting the excipient have relatively high water absorption rates, and the excipient itself is likely to be deformed.
Furthermore, as mentioned in the third problem above, if a crack is created on the surface of the excipient covering the fibers of the fabric, moisture may enter through the crack. As a result, the fibers of the fabric may absorb moisture so as to be stretched, causing the molded damper to be deformed, whereby the properties of the loud speaker are unfavorably affected.